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What's the "Super Moon" about?

Topic: Moon
Article Published - July 27, 2014
by Dr. Jim Lattis, Director, UW Space Place
August SuperMoon

We'll soon be hearing about the super moon of August, which will occur on the 10th of that month, and on that evening, if the sky is clear, we'll be able to see the full moon amidst the stars of Aquarius, as shown above. But what does it mean to call the full moon a "super" moon? In fact, the super moon is what we once called "full moon" in the days before escalating media hype. We get a full moon, of course, about every 29.5 days, so that hardly makes news. And we also know that when we see the full moon near the horizon, just after rising or just after setting, it seems dramatically larger than when we see it high in the sky. We call that the "moon illusion," and it is well known, but it isn't the super moon.

The idea of the super moon stems from the orbit of the moon itself. Like most orbits in the solar system, the Moon's orbit is not circular but elliptical. The elliptical orbit causes the Moon's distance from Earth to vary as it completes each orbit, from its closest approach, called perigee, to its greatest distance, called apogee. Of course, when the Moon is closer to us, it will appear larger, when more distant, it will appear smaller. At closest approach (perigee), the Moon's apparent size will be a maximum, and at greatest distance (apogee), its apparent size will be at a minimum. Perigee does not always coincide with the day of the full moon (perigee and apogee can occur at any phase), but when it does, the full moon will seem as big as it can be, and this has come to be called the super moon. Conversely, when apogee coincides with full moon, the moon will appear as small as it can possibly be, and this is sometimes called the "micro moon."

So how super is the August 10 super moon? At the moment of full moon, which is less than a half hour from the moment of perigee, the Moon's distance from the center of the Earth will be 356,896 kilometers. At the full moon of September 9, the Moon's distance will be 358,387 kilometers, or about 4% farther away.

How big a difference will this make? The easiest way to visualize the situation is to make a scale model. Actually doing it yourself is best, but here's the general idea. Let Earth be a small marble. Then the Moon will be about the size of a peppercorn. Place them about 14 ˝ inches apart. That's the average radius of the Moon's orbit in our model. (see image) For apogee, move the peppercorn about an inch farther than average from the marble—that's the micro moon. To show the Moon at perigee, where it will be on 10 August, move the peppercorn about an inch closer than average to the marble. For the 9 September (not-quite-super) full moon, move the peppercorn about one-sixteenth of an inch farther away from the marble. Not much difference, eh? But the Moon's distance can vary more than that, of course. Let's compare to an average-distance full moon. How much larger (than average) does the peppercorn look at perigee when seen from the marble? The answer is, it looks a bit larger, but not a great deal. Is it a "super" peppercorn at perigee? Try it and see.

The greatest difference in size, of course, is between the apogee and perigee full moons. If we photograph the full moon at apogee and perigee (micro and super moons), we can easily see the apparent difference. (Here's a good example: The difference between the near-perigee full moon (super moon) of May 2012 and the near-apogee full moon (micro moon) of November 2012 is shown here. (see image)

The difference is certainly apparent, but we never get to see them side by side in the sky. When we see the full moon overhead, even a super moon, it is surprisingly small—your thumb at arm's length will easily be two or three times wider than even a super moon. So by all means do go out and look at the full moon on the evening of August 10, and remember that you're seeing it quite near its closest possible distance. But if it doesn't look particularly "super," don't worry, you didn't get the date wrong.

Aside from the fuss over super and micro moons, it turns out that the coincidence of perigee and apogee with full and new moons does have interesting, important, and visible effects on Earth, but the only observers who are likely to notice are those who live near the sea, because the effects are on the ocean tides. The ebb and flow of the tides results from the gravitational forces of the Sun and Moon, which can combine either to reinforce or counteract each other. At full and new moon, the forces combine to increase tidal amplitude (making high tide higher and low tide lower than average), and such tides are called "spring" tides (which, by the way, have nothing to do with the season we call spring). Conversely, at first and last quarter moons, the solar and lunar tides counteract each other reducing tidal amplitudes (lower highs and higher lows), which are called "neap" tides. If lunar perigee coincides with either full moon (which would then be your "super" moon) or new moon (which is usually invisible in any case), then the high spring tides at that time will be even higher. (The situation can be exacerbated still further if Earth, in its orbit, is near perihelion, thus enhancing the solar component of the tide.) Such high tides can cause problems if, for example, a major storm, such as a hurricane, is also raising sea levels, in which case a perigee spring tide might enhance flooding hazards. The opposite extreme case would be the minimal tides that would happen at an apogee neap tide. So the combination of apogee or perigee with particular lunar phases can certainly have important terrestrial effects, but the appearance of the Moon in the sky is not one of them.

While on the subject of lunar phenomena, watch this space for information on the lunar eclipse of 8 October 2014. It will probably be hyped as a "blood" moon. Oh well.

From the University of Wisconsin's "Space Place"
Villager Mall, 2300 S Park Street, Madison

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